Prior art workers have devised many types of pneumatic fastener driving tools. In its most usual form, the pneumatic fastener driving tool has a manual trigger and a safety, both of which must be actuated in order for the tool to fire. The most usual type of safety comprises a workpiece responsive trip which contacts the workpiece and is shifted thereby. Most frequently the workpiece responsive trip disables the manual trigger, unless the trip is pressed against the workpiece. An example of such a tool is taught in U.S. Pat. No. 3,278,106.
Prior art workers have also devised a number of additional safety means for such tools. As an example, U.S. Pat. No. 3,964,659 teaches a pneumatic fastener driving tool provided with safety means introducing into the firing sequence of the firing control means a time limit within which both the manual trigger and the trip must achieve their firing positions. If the manual trigger and the trip do not achieve their firing positions within the time limit, the firing sequence must be reinitiated with a selected one or both of the manual trigger and the trip initially in their normal positions.
In addition, pneumatic fastener driving tools have been developed which are provided with a "Auto-Fire" mode of operation wherein the operator can drive a plurality of fasteners by simply pulling the trigger and moving the fastener driving tool along the workpiece. U.S. Pat. No. 3,278,104 teaches such a fastener driving tool.
The pneumatic fastener driving art has achieved a high degree of sophistication. However, the more sophisticated pneumatic fastener driving tools have become, the more complex they have become and more expensive to manufacture. Various types of mechanical safeties have critical timing. If their parts become worn, their safety features are lost. Some mechanical safety features can be bypassed by the operator. For example, operators have been know to wire a workpiece responsive trip in its depressed position. When this is done, the safety feature provided by the workpiece responsive trip is lost. Pneumatic safety devices are vulnerable to a faulty or nicked O-ring, or to a sticking valve.
The present invention is based upon the discovery that if a pneumatic fastener driving tool is provided with an electronic control system, it can be greatly simplified in construction, and complex valving and mechanical linkages can be eliminated. A pneumatic fastener driving tool having an electronic control system is more reliable, less expensive to manufacture and more versatile. Should the tool fail for lack of electrical power, it will fail in a safe mode. Pre-fire (firing of the tool upon connection to a source of air under pressure) is discouraged since the air gets to the volume above the firing valve more directly. The electronic control system lends itself well to an auto-fire mode of operation without complex valving. The auto-fire feature can be made adjustable, say over a range of from 1 to 20 cycles per second, for example. Furthermore, the control circuit may have a number of input signals in addition to those provided by the trigger and the safety. For example, an input signal may be provided from an empty magazine sensor to prevent dry firing. An input signal may be provided from an overpressure sensor, disabling the tool if the air under pressure is at too great a pressure. Furthermore, the control circuit itself can be designed with various built-in safety features. For example, the circuit can be designed to prevent cycling of the tool if the safety and trigger are not both activated within a predetermined time limit. An input signal may be provided to prevent cycling of the tool should the pressure of the air be too low.
The control circuit may be pre-programmed at the factory to establish a desired mode of operation of the tool. The tool could be provided with a control circuit enabling the operator to select one of a number of modes of operation. As yet another alternative, the operator could change the tool from one mode of operation to another by simply replacing one control circuit (in the form of a chip or the like) with another. Finally, the electronic control system of the present invention provides extra design flexibility for component placement within the tool.